1. Field of the Invention
The present invention relates in general to waste material disposal and in particular to the disposal of reclaimed waste dust that contains undesirable chemical compounds, such waste dust including cement kiln dust (CKD) and alkaline bypass dust (ABD) that is generated during the formation of cement clinkers in a cement making process using a kiln. The preferred kiln is of the type such as a rotary kiln or a vertical kiln. The waste dust may be utilized to form cement clinker or to make lime.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In the production of Portland cement clinker, there are certain undesirable chemical compounds collected in Cement Kiln Dust (CKD) and Alkali Bypass Dust (ABD) such as chlorine, sulfur, and alkali and other various precipitated chemical compounds as is well-known in the art. These chemical compounds are volatilized into waste gases in the burning zone of a high temperature (approximately 1500 degrees C or 2732 degrees F) rotary kiln and are recovered with pollution control equipment as kiln dust (CKD and ABD). Depending upon the process and the chemistry of the raw material and fuel, a portion of the kiln dust can be returned to the kiln feed and processed into Portland cement clinker. Normally, most of this kiln dust is wasted and must be placed in a landfill. Some kiln dust is used for fertilizer and soil stabilizer depending on its chemical composition.
Commonly assigned U.S. Pat. Nos. 5,494,515 and 5,421,880 disclose processes for using crushed and screened blast furnace slag and steel slag, respectively, to form cement clinkers. During such processes, as disclosed therein, temperatures in the kiln are approximately 1500 degrees C (2732 degrees F). At these temperatures, well-known chemical processes occur to form cement clinker that exits the heat end of the kiln and that generate waste gases containing feedstock dust and precipitated undesirable chemical compounds as set forth above.
Because of the nature of these waste gases, government regulations require that pollution control equipment be installed to remove the dust containing the undesirable precipitated chemical compounds to a level sufficient to meet government standards. These pollution control devices can generate hundreds of tons of waste dust every day that must be reclaimed and a location found for its disposal. The amount of waste dust generated depends upon the production capacity of the cement making plant. Disposal of the reclaimed waste dust is usually done by means of landfills or stock piles. If one assumes that a plant can generate 300 tons of waste dust a day 365 days a year, then there must be a land fill capable of storing approximately 110,000 tons of reclaimed waste dust for each year of cement clinker production. In 10 years, over one million tons of reclaimed waste dust must be stored somewhere for each average cement plant.
Waste dust is defined herein as the cement kiln dust (CKD) and alkaline bypass dust (ABD) recovered from the exhaust gases generated in a cement clinker manufacturing process with the use of well-known pollution control equipment associated with rotary kilns or vertical kilns.
Further, various government entities are looking at reclaimed waste dust as an environmental problem that must be solved. Thus, attempts are being made to reduce the level of emissions of the undesirable precipitated chemical compounds that are found in, and contribute to, huge masses of waste dust that must be reclaimed or stockpiled.
U.S. Pat. Nos. 4,173,487; 4,584,022; and 5,782,973 relate to methods for processing kiln dust at high temperatures in specially designed kilns, where the volatile materials are separated by heat, collected in scrubbers, and can be sold as products such as fertilizers. Any remaining material is usually returned to the kiln as kiln feed.
One such method such as that disclosed in U.S. Pat. No. 5,782,973 is a process for the recovery of cement plant alkali by-pass dust (ASD) utilizing a fluid bed reactor system. The method attempts to recover 90% of the kiln dust as a usable, low alkali, cement product in addition to an alkali-salt product that has a commercial value as a source of potassium for fertilizer use. It is said that, in this process, the low alkali cement can be blended into higher alkali rotary kiln products to produce a controlled alkali level end product. It requires a feed of 4.times.20 mesh pelletized raw material. The pellets are formed with coal or coke and ABD and are injected into a counter flow preheat shaft and fall through the combustion gases. The pellets are rapidly preheated and partially calcined. They then enter a fluidized bed with up to 90% of the process coal or coke fuel required being formed as a part of the pellets to bring the carbon content of the final raw material to a point between three and nine percent by weight. The pellets are then fired and the cement clinkers are formed. The NO.sub.x and SO.sub.x emissions are said to be less than conventional systems due to the low temperature combustion (2012 to 2552 degrees F--1100 degrees C to 1400 degrees C) that takes place in the fluidized bed and the use of natural gas as heating fuel. This system uses a well-known reducing atmosphere wherein the oxygen in the coke or coal is used as a heating element and, during combustion, generates a gaseous sulfur and alkali by-product. Some of the compounds in the by-product are K.sub.2 SO.sub.4, Na.sub.2 SO.sub.4, and may contain other compounds. The potassium and sodium compounds are alkali products that have a commercial value as a source of potassium for fertilizer use.
Hence, newly generated waste dust still remains as an alkali dust product that is used as a fertilizer. Thus there remains the problem of disposal of the by-products of the cement making process. With this described prior art system, a fertilizer business must be operated in parallel with the cement clinker operation.
In another prior art system, the waste dust from a kiln is reclaimed and used in that same kiln as a feedstock material in a process for making cement clinker The high temperatures dissociate, or volatilize, up to 100% of the undesirable compounds in the reclaimed waste dust such as chlorine, sulphur, and alkalis, and cement clinkers are formed. Also, in this process, the undesirable volatile compounds in the CKD material (as well as those in the added feedstock material) are once again dissociated by the high temperatures and simply create new waste dust that must be reclaimed and the disposal problem remains. In addition, the high percentage of the volatile materials in the reclaimed waste dust (that is added to the input feedstock material entering, the feed end of the traditional rotary cement kiln) causes the feedstock material within the kiln to become sticky and creates undesirable plugging and mud ring or clinker buildup. Thus, the use of the waste dust restricts the normal flow of raw feedstock materials by the formation of mud rings and clinker rings inside the wet or dry rotary kiln. In addition, in the dry process, material builds up in the preheater/precalciner and causes plugging. These problems cause production losses, burning problems, material quality problems and do not resolve the environmental problems (in both the air and in landfills). See also U.S. Pat. Nos. 5,219,544; 5,437,721; and 5,919,301.
There are some commercial uses for CKD and ABD but the market is limited and the excess has to be placed in a landfill.
It would be advantageous to have a waste dust disposal system that not only used the reclaimed waste dust in a process for making hydraulic materials such as cement clinkers or lime but which also used all of the undesirable chemical compounds in the reclaimed waste dust, including newly generated waste dust so that the disposal problem is eliminated.